1. Field of the Invention
This invention relates to the formation of a metal layer on a substrate. More particularly, the present invention relates to the formation of defect free or substantially defect free metal layers in an integrated circuit.
2. Description of the Related Art
Defects incident to the deposition or formation of metal layers on a substrate of, for example, an integrated circuit decrease the reliability of the resulting device, decrease the production yield and increase costs. Some defects, variously known as whiskers, spikes, hillocks or nails (hereafter collectively referred to as whiskers), have been observed incident to the formation of aluminum containing metal alloys, among others. Whiskers often manifest themselves as three-dimensional projections or non-uniformities in the deposited or formed metal layer. Conventionally, Chemical Vapor Deposited (CVD) W-plug is used to fill vias and contacts to connect different metals levels in IC designs. After this step, Chemical Mechanical Polishing (CMP) must be used to remove the W from the entire wafer surface. However, CMP techniques can add many steps to the semiconductor manufacturing process, and are poorly controlled. Thus, CMP has not proven to be an entirely successful method of overcoming the effects of such whiskers and other unwanted topographic defects on the surface of metal layers.
CMP has not always been necessary to correct problems associated with whiskers, as the size of some whiskers and other like defects has not always necessitated its use. Indeed, in defects of small size, the presence of such whiskers may not appreciably affect either the function of the devices or the overall yield of the process. However, as line widths decrease below about 0.50 microns, for example, the size of such whiskers often becomes an appreciable fraction of the line width and may, in fact, be of a size that interferes with the proper functioning of the semiconductor device.
Such a situation is depicted in FIG. 3. As shown in FIG. 3, semiconductor devices often include a plurality of electrical conductors, such as conductors 310. When a defect, such as a whisker 320 is of a size that is comparable to the width of and/or the distance between adjacent conductors (the pitch), the whisker may create an unwanted electrical pathway between two or more adjacent conductors, causing at least a partial short. The whiskers are believed to be single crystals in structure, formed as an extrusion of a1, for example, as a result of compressive-stress generated secondary nucleation. These whiskers are embedded into the film have different etch rate and serve as bridges in the form of xe2x80x9cnailsxe2x80x9d after the metal lines are formed.
Whiskers are a common problem in at least aluminum layers and in aluminum containing metal layers. Considering now FIG. 1, a method of depositing an e.g., aluminum layer within, for example, a high aspect ratio via, includes a first step of depositing an adhesion layer onto a silicon substrate. The adhesion layer may include titanium, for example, and the deposition process may include collimating of the titanium source to insure uniform step coverage of the adhesion layer within high aspect ratio vias, as shown in FIG. 1 at step S0. As shown in step S1, a first metal layer may be deposited at a relatively low temperature, followed by a xe2x80x9chotxe2x80x9d second metal layer deposition step at a relatively higher temperature, as shown in step S2. Finally, a so-called xe2x80x9ccapxe2x80x9d layer may be formed, at a relatively high temperature and power, as shown at steps S3. The high deposition temperature of step S2 allows the deposited metal to flow easily into, for example, high aspect ratio vias and other like structures. Therefore, to flow easily into high aspect ration vias, the e.g., aluminum or aluminum-containing alloy must be deposited close to its critical ratio of growth or deposition temperature versus melting point. A critical ratio of unity means that the growth or deposition temperature is equal to the melting point of the metal to be deposited or grown. To fill such high aspect ratio vias requires the metal to be deposited or grown at a critical ratio that is somewhat close to unity. The deposited metal, under the described conditions, is allowed to flow into vias, contacts and trenches, i.e., the temperature should be at least higher than that required to cause plastic deformation of the xe2x80x9chot metalxe2x80x9d.
Investigations into the physical phenomena behind the formation of whiskers and other similar defects have led to a belief that such defects may be due to the relaxation of compressive and tensile forces within the aluminum or other metal layer formed or deposited by such a regime. Indeed, because the aluminum may be in a highly compressive state, due primarily to high temperature and/or high growth rate, a secondary nucleation phenomenon may occur in which secondary nuclei may form to release some of the compressive forces, which secondary nuclei have a much faster growth rate than the surrounding layer or layers. Such secondary nucleation may be at least one of the root causes of the formation of whiskers, and other similar defects. Other models point to grain boundary diffusion as the primary whisker formation mechanism and predict lower whisker density in strongly oriented aluminum films.
Various methods have been proposed to reduce the occurrence of such whiskers, but none are believed to effectively address the problem without, however, adding a plurality of additional unwanted steps to the process, increasing the cost of the resultant devices, increasing processing time and/or decreasing the yield of the process. What are needed, therefore, are methods for depositing a metal layer or layers on a substrate that do not suffer from performance or yield degrading whiskers and like defects, or methods that at least decrease the occurrence and/or size of such whiskers and other similar defects. What are also needed are methods for depositing a metal layer(s) onto a substrate that minimize the number of additional steps that must be carried out to achieve a defect-free or substantially defect free metal layer on a substrate, such as a semiconductor substrate.
An object of the present invention, therefore, is to provide methods for depositing a metal layer or layers on a substrate that do not suffer from performance or yield degrading whiskers and like defects, or methods that at least decrease the occurrence and/or size of such whiskers and other similar defects. Another object of the present invention is to provide methods for depositing a metal layer(s) onto a substrate that minimize the number of additional steps that must be carried out to achieve a defect-free or substantially defect free metal layer on a substrate, such as a semiconductor substrate.
In accordance with the above-described objects and those that will be mentioned and will become apparent below, a method depositing a metal layer onto a substrate, according to an embodiment of the present invention, comprises the steps of
depositing a first metal layer at a first deposition temperature;
depositing a second metal layer on the first metal layer at a second deposition temperature higher than the first deposition temperature;
reducing at least one of a growth rate and a temperature of at least the second metal layer; and
depositing a third metal layer on the second metal layer.
According to other embodiments, a step of depositing an adhesion layer on the substrate before the first metal layer is deposited may also be carried out. The first, second and/or the third metal layers may include one or more of the following elements: aluminum, copper, tungsten, molybdenum, tantalum, titanium and alloys thereof. The second temperature may be sufficient to allow the second metal layer to flow into high aspect ratio vias. The second temperature may be selected so that a ratio of the second temperature to a melting temperature of the second metal is between about 0.65 and about 0.95. The reducing step may reduce the growth rate substantially to zero. The temperature in the reducing step may be reduced to below about 400xc2x0 C. Alternatively, the temperature in the reducing step may be reduced to below about 250xc2x0 C. The reducing step may be carried out by removing the first and second metal layers from a process chamber in which the second metal depositing step is carried out.
According to another preferred embodiment, a method of forming a metallization layer on a substrate, according to the present invention, comprises the sequential steps of:
forming a first metal layer on the substrate at a first temperature in a first process chamber;
forming a second metal layer on the first metal layer at a second temperature in the first process chamber;
moving the substrate to a second process chamber for a period of time sufficient to inhibit a formation of defects upon subsequent formation of a third metal layer; and
forming a third metal layer on the second layer.
According to further embodiments, the second process chamber may be at a third temperature, the third temperature being lower than the second temperature. The third metal layer may be formed at a higher power than is used to form the second metal layer and the third metal layer may be formed at a fourth temperature, the fourth temperature being lower than the second temperature. A step of returning the substrate to the second chamber prior to the third metal layer forming step may also be carried out. The heater within the second chamber may be turned off prior to the third metal layer forming step.
The present invention may also be viewed as a method of forming a substantially smooth and substantially defect free metal layer, comprising the steps of.
forming a first metal layer;
forming a second metal layer on the first metal layer at a temperature wherein a critical ratio of deposition temperature to melting temperature is between about 0.65 to about 0.95
stopping the second metal layer forming step and cooling the first and second metal layers to a temperature below which the second metal layer ceases to flow; and
forming a third metal layer on the second metal layer at a temperature wherein the critical ratio is below about 0.65.
According to other preferred embodiments, the first, second and/or third metal layers may include one or more elements selected from the group consisting of aluminum, copper, tungsten, molybdenum, tantalum, titanium and alloys thereof The stopping and cooling step may be carried out by removing the first and second metal layers from the location where they were formed. The third metal layer forming step may be carried out in the location where the first and second metal layers were formed. The first, second and third metal layers may include aluminum, and the second temperature may be selected between about 450xc2x0 C. to about 500xc2x0 C.